When severe hip joint problems are encountered, it is sometimes necessary to replace the hip joint, either the ball or the socket or both. The large upper leg bone, or femur, has a long lower main portion, with a head or ball connected by a neck portion angled inward toward the hip socket from the upper end of the main portion of the femur.
One generally used hip joint replacement technique involved removal of the head and neck of the femur, and the insertion of a long angled and tapered metal prosthesis into the central "intramedulary" canal at the open upper end of the main straight portion of the femur. This femoral prosthesis typically has a relatively small metal ball at its upper end which mated with small plastic socket mounted on the hip side of the joint.
This "total" hip replacement technique was drastic, involving complete removal of the head and neck of the femur, and made any subsequent hip joint problems difficult to handle.
On the socket side of the joint, referred to as the "acetabular" in medical parlance, some prostheses were employed which used a plastic cup to mate with the femoral component; and it has been determined that these plastic acetabular components were subject to considerable wear, producing particulate matter which adversely affected the lifetime of the hip joint prosthesis. Metal-to-metal joints were also proposed in the 1960s, but lack of accuracy in sphericity and other problems had prevented their wide acceptance.
As shown in U.S. Pat. No. 4,123,806, granted Nov. 7, 1978, an early femoral prosthesis involved a cobalt-chromium-molybdenum metallic shell of generally hemispherical shape, designed on the principle of removing all non-viable femoral head bone, but also preserving as much of the head and neck as possible. A polyethylene socket or acetabular component was employed. The femoral shell was cemented onto the head of the femur, following shaping to one of several standard sizes. This surface replacement conserved bone and permitted a full femoral hip replacement if problems arose with initial replacement prosthesis. However, in several cases the polyethyl-ethylene wear and resultant particulate material caused loosening of the femoral shell and/or the acetabular shell.
In a more recent prior art development, as described in a publication entitled "Femoral Surface Replacement System, Surgical Technique," a metal-to-metal hip joint prosthesis has been employed, using a relatively thin all-metal socket prosthesis secured in place by bone-ingrowth; and a cemented metal shell as the femoral component. The metal shell femoral component was provided with a central tapered stem extending centrally from the shell into the femoral neck for a distance slightly less than the diameter of the shell.
However, the results have, on occasion, not been quite as good as would be desirable, and occasional problems have arisen with regard to accurately positioning the femoral shell, and providing the very uniform layer of cement between the femoral head and the metal shell, which is desirable for firm securing and long life of the prosthesis and/or for preventing notching of the neck which could lead to femoral neck fracture.
It is also noted that metal-to-metal hip joint prosthesis have been tried heretofore, but have not been entirely satisfactory, with clicking and ratcheting noises occurring in some cases, and with the potential increased torque causing loosening.
Further, in some cases, the original length of the leg has not been maintained, with prior art prostheses which have been employed.
Concerning one other prior art prosthesis involving surface configurations on the prosthesis stem for encouraging bone ingrowth, and no cement on the stem, the result was secure fixation of the stem, but atrophy of the bone around the outer periphery of the femoral head.